Stabilizing cellulose esters



- Patented Oct. 27,1942

. zsowiao s-ramnzmc caummosa asrsas Ferdinand Schulze, Waynesboro, Va... winter to y E. L du Pont de Nemours & Company, WilmingtonADeL, a corporation of Delaware No Drawing. Application March '1, 1940, Serial No. 322,763

ftciaims.

The present invention relates to a new and improved process ior the production of cellulose acetate, and other organic acid esters or cellulose, which will be exc'eedinglystable and suitit being understood that it will be equally applicable to the production of other organic acid esters of cellulose.

(01. sca -23o) In the currently used commercial manuiacturing' processes, it is customary to acetylate cellulose with acetic anhydride, in the presence or acetic acid as the solvent and a quantity of suliuric acid as the catalyst. Sulfuric acid is usually present in the acetylation mixture to the extent or from 3% to 10%, basedon the weight oi the air-dry cellulose. The resultant cellulose triacetate solution is treated with a quantity of aqueous acetic acid and an additional quantity oi sulfuric acid to hydrolyze the product to an acetone-soluble material. As a consequence of the use or sulfuric acid in the production of the cellulose acetate, sulhmcontaininc radicals, for example, sulfate radicals, remain as an integral part oi the flnalproduct. The amount oi sullate combined with the cellulose acetate may be only from 0.03% to 0.07% (calculated as HBSOl),

but in spite of its relatively minute concentration, it has far-reaching detrimental sheets on .the physical and chemical properties oi the cellulose acetate. It is well known, ior example, that the stability to heat of cellulose acetate is seriously'diminished by the presence oi a small amount of combined sulfates. Uomblned suliates also influence the stability of the cellulose acetate to hydrolysis by water. Instability oi cellulose acetate to heat is nianiiested by darkeninc and embrittlement of manufactured articles such as molded plastics or film. Instability to hydrolysis causes embrittlement oi the manufactured-articleahnd in photographic him base it has a serious effect on the photocraphic emulsion.

n has been determined that the combined suliute in cellulose acetate is in the form oi a hull ester oi sulfuric acid. when cellulose acetate containing the combined sulfate hall ester is washed with hard water, metal cations displac the hydrogen of the sulfate hall ester, that is,

apists halt ester becomes neutralized with cations such as calcium or masnesium which of this neutralization, the stability of cellulose acetate to heat and hydrolysis is greatly improved and for that reason it has in the past been considered necessary and desirable to wash cellulose acetate with hard water, or to actually add additional mildly alkaline agents such as sodium bicarbonate, calcium carbonate or magnesium carbonate to the mixture during washing. This method of stabilization products a product that is satisfactory for many industrial uses.

In the development of cellulose acetate for molding powder, it was at ,first assumed that the discoloration of cellulose acetate compositions during molding was caused by the presence of combined sulfate halt esters and could be eliminated by the neutralization oi the sulfate halt esters with calcareous matter. it has now been i'ound, however, that in the presence oi an excess quantity of calcareous matter in cellulose acetate, the discoloration actually becomes worse. It has furthermore been found that discoloration of cellulose acetate compositions durinc molding can be minimized by reducing, or eliminating, this excess quantityoi calcareous mineral matter. v

The discoloration oi molding powder is iound to be a two-stage phenomenon. presence of calcium, magnesium, or sodium salts causes discoloration to a yellow or light brown color at relatively low temperatures, ior example irom 150 to 200 0.. At higher temperatures, ior example from 200 to 300 0., another type oi discoloration sets in which is caused by combined sulfate in the cellulose acetate. It appears that 1 low temperature discoloration oi cellulose acetate are normally present in'hard'water. as a so molding powders is caused by substances having an alkaline reaction, whereas high temperature discoloration is caused by substances having an acid reaction.

cellulose acetate molding powders are emplayed for both compression and injection molding. it is, therefore, customary to test samples of molding powder for stability by making test molded pieces by both. compression and injection methods. Because compression moldinu is usually carried out at a lower temperature than injection molding, the former will iiulicate whether or not the cellulose acetate is contendnated with calcareous mineral matter, while the latter test will indicate the presence or absence oi objectionable amounts of combined sulfate.

From the above discussion, it is apparent that to be satisfactory tor-use inmoldinz powder, cellulose acetate must contain a minimum of in general, by incorporating, in cellulose acetate containing sulfuric acid half esters, alkali metal or alkaline earth metal salts, such as calcium, magnesium or sodium salts in an amount exceeding that necessary for the neutralization, by the ionized metal cations, of the sulfuric acid half esters, and then carefully removing only the excess salts from the cellulose acetate.

Example I One hundred parts of air-dry cotton cellulose are treated with 620 parts glacial acetic acid.

After mixing for 45 minutes, there is added a quantity of acetic anhydride molecularly equal to the quantity of water present in the mixture, and an additional quantity of 7 parts of acetic anhydride. Fifteen minutes later there is added a solution containing 175 parts of glacial acetic acid, 7 parts of acetic anhydride, and 1.6 parts of sulfuric acid. In this mixture, the sulfuric acid has been converted into acetyl sulfuric acid by reaction with acetic anhydride. (This mixture in preference to one containing no acetic anhydride is used, because it has been found that acetyl sulfuric acid produces cellulose acetate of somewhat lower combined sulfate content and, therefore, of better stability.) The temperature of the mixture rises somewhat after the addition of the catalyst, but is maintained at or below 26 C. by means of cooling water. After 3 hours total time, the temperature has been brought back to 20 C. and 27 parts acetic an-v hydride -are then .added and one hour later an additional 210 parts acetic anhydride areadded. The temperature is then allowed to rise to about 55 C. and when acetylati on is completed, as is evidenced by complete solution, 210 parts of 60% acetic acid are added. The temperature of the solution is raised to 60-65 C. and about two hours later another portion of 210 parts of 60% acetic acid is added. The mixture is saponified at a temperature between 60 C, and 65 C. for ten to fifteen hours, or until the combined acetic acid of the cellulose acetate is reduced to about 56.5%. When this point is reached, 750 pounds of 18% acetic acid preheate to 65 C. are added to the mixture, and the mixture held at 65 C. for an additional period of approximately ten hours, at the end of which time the combined acetic acid has dropped to between 54% and 55%. The solution is then filtered and precipitated in 2,500 parts of water. The precipitated cellulose acetate contains between 0.010% and 0.015% combined sulfate. I

2,000 pounds of cellulose acetate prepared in this manner are washed with hard water containing 80 to 100 parts per million of hardness, expressed as calcium carbonate, of which 20 to 30 parts are magnesium carbonate, until the acidity-has been alkalinity of the eiiluent wash water is substantially the same as that of the lnnuent water. The

completely eliminated and thesoon as the mineral salts mixture is next washed with demineralized water or distilled water which is passed into the cellulose acetate at the rate of 200 pounds per minute until 30,000 pounds of water have passed through the mass. The cellulose acetate is then dried.

Example II previously, for instance, by means of a suitable indicator, or by electrometric methods, and the pH of the eflluent water should'be at least 4.5. To this cellulose acetate which is in a slurry with 40,000 parts of water, there is added 0.4 part of solid magnesium carbonate and the slurry stirred for hour. Following neutralization, the slurry is washed with distilled or demineralized water at the rate of parts of water per minute for three hours, The acetate is then drained and dried.

' In case the tap wash water normally available for washing the cellulose acetate is relatively low in magnesium ions, magnesium salt may be added, such as, for instance, from 20 to 50 parts of magnesium carbonate ter. Instead of using hard water as provided by nature, substantially mineral-free water to which has been added the desired quantity of magnesium carbonate or an equivalent material may, of course, be used. In the modification described under Example II, the amount of solid magnesium carbonate which is added to the washed cellulose acetate may be varied quite widely, for instance, between 0.2 and 2 parts per 2,000 parts of cellulose acetate, and preferably between 0.4 and 1 part per 2,000 parts of cellulose acetate. It is, of course, desirable not to add any great excess of magnesium salt, since this means that more washing with mineral-free water following this addition will be needed. In place of magnesium carbonate other magnesium salts such as magnesium acetate or magnesium citrate may be used. Other alkaline earth metal or alkali metal salts of weak acids may also be used.

Generally, the washing following, neutralization may be varied from 1 to 5 hours, depending on the amount of salt introduced and the rate of washing. In all cases the rate and time of washing are preferably adjusted so that the original solution on the cellulose acetate is essentially displaced with fresh mineral-free water. Care must be taken, however, that the cations which have attached themselves to the sulfate radicals of the cellulose acetat are not removed by the subsequent washing with mineral-free water. The washing with mineral-free water for the removal of mineral salts should be discontinued as have been substantially removed. It hasbeen found that if the washing is continued for a substantial period of time after the mineral salts have been removed, the cations per million parts of wa- 0.05 and 0.2 pound of water per minute per pound of cellulose acetate, and preferably between 0.1 and 0.15 pound per minute per pound of cellulose acetate. Of course, it may be possible to wash with a smaller quantity of water than above set forth if the washing is carried out at a very low rate, or a larger quantity of water may be used if the washing iscarried out at a very high rate. The amount of wash water used and the rate of washing must be so regulated that the washing is discontinued substantially at the time when the excess mineral salts have beenremoved. This can be determined by titration with well known indicators, or by determiningthe pH of the eiiluent wash water by known electrometric methods.

Preferably, the method of the present invention is combined with the processor acetylation which will produc a cellulose acetate containing no more than about 0.015% radicals.

If desired, the cellulose acetate may bebleached. for instance, with a-diiute solution of potassium permanganate followed by discharge with a weak oxalic acid olution and further-"Z 0 washing.-

In the copending application of Ferdinand Schulze and Merle A. Heath illed of even date. herewith, there is disclosed a method for the acetylation of cellulose in the presence of sulfuric acid as the catalyst, which sulfuric acid is present'in sufllcient an1ount to carry out the acetylation but-in no case exceeds 2% of the weight of the air-dry cellulose entering the reaction, and no additional amount of'sulfuric acid 40 or other catalyst is added to the cellulose acetate during the hydrolysis thereof.

combined sulfate 25 By this. process it is possible to produce a celthroughout the specification refers to waterhaving no more than 10 parts of mineral residue, nor more than 0.2 part of alkaline earth metals, per

million parts of water and having a pH value of.

at least 5. Until recently, distilled water was the only type of water generally suitablefor this purpose and-this would be far too expensive to be practical. Fortunately, it is now possible to obtain water having a high'degree of purity, a pH value about 5, and freedom from mineral salts by quite simple and inexpensive methods. Cations may be removed from the water by percolating it through a bed of water-insoluble resin which contains free acid groups. with the acidic resin with the liberation of hydrogen ions. The metallic salts originally present in the water are thus converted to the corresponding acids. The eiiiuent fromthe cation-accepting imit is then passed through a basic resin containing amino groups which combine with the acid present in the water with the result that the eilluent'from the two combined units is virtually free of mineral matter. The two units can be regenerated after they are exhausted by treating them respectively with solutions of strong mineral acid and strong alkali; The water ob-' Efliucnt Eflluent 33: from from mm: cation anion acceptor acceptor Total residue 72 0. 0 8. Loss oni itionit 1.0 i. Fixed res due 57 8. 0 7. 4d Alkalinity 61 "(2.0 2. sulfates i 2 2 v 0. Hosp hardness it 0 0 lose in such a manner that it will contain no more than. about 0.015% of combined sulfate radicals, and then neutralizing the remaining this combination the freshly precipitated cellulose acetate, prepared in accordance with any of the conventional processes for the acetylation of cellulose, and containing from .03% to .07% combined sulfate radicals, is first washed with mineral-free water until the emuent water contains no more than .1% acetic acid. If the so-washed cellulose acetate is then subjected to the action of wet steam under. a pressure of about pounds per square inch for a period of less than one hour to desulfur the same, the combined sulfate radicals in the resulting cellulose acetate may be reduced to only 0.01%. In this case the remaining combined sulfate radicals may then be neu- Since it is obvious that many changes and modifications can be made in the details above described without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to these de-' tails except as set forth in the appended claims.

I claim:

l.- The process of producing hig y stable organic acid esters of cellulose which comprises essentially washing a freshly precipitated organic acid ester of'cellulose containing combined sulfate radicals with water containing a salt from the group consisting of alkali metal and alkaline earth metal salts whereby to neutralizethe sulfate radicals with cations from the group consisting of alkali metal and alkaline earth metal cations, then removing excess mineral salts from the cellulose ester by washing the same with no more than 30 pounds and no less than 5 pounds of mineral-free water per pound of ester, the latof between 0.05 pound and 0.2 pound per minute Der pound of ester, said mineral-free water having no more than 10 parts of mineral residue,.

cations combined tained from a softening system of this type may ter washing operation being carried out at a rate 2. The process of producing h hly stable cellulose acetate which comprises essentially washing a freshly precipitated cellulose acetate containing combined sulfate radicals with water containing a salt from the group consisting of alkali metal and alkaline earth metal salts whereby to neutralize the sulfate radicals with cations from the group consisting of alkali metal and alkaline earth metal cations, then removing excess mineral'salts from the cellulose acetate by 10 washing the same with no more'than 30 pounds and no less than 5 pounds of mineral-free water per pound o'i ester, the latter washing operation being carried out at a rate of between 0.05 pound and 0.2 pound per minute per pound of ester, said mineral-free water having no more than 10 parts oi mineral residue, nor more than 0.2 part of alkaline earth metals, per million parts of water and having a pH value of at least 5.

. FERDINAND SCHULZE.

CERTIFICATE 01 co men on.

Patent No. 2,500,1 0. October 27, 19142.

FERDINAND scHULzE It is hereby certified that error appears in the printed epecification of the above numbered patent requiring correction as follows: Page 1', sec- 0nd column, line 9, for "prodlcts' read --producea--; page 5, second column, line 15, for about read --aboveand that the said Letters Patent should be read with this correction therein that the same inay conform to the record of the case in the Patent Office. I I

Signed and sealed this 1st day of Decanber, A. D. l4.2;

Henry Van Arsda'ie, (Seal) Acting Commissioner of Patents. 

